Differential pistons are pistons in piston-cylinder arrangements with pressure being applied to opposite sides of the piston. They are used either to displace the differential piston in two opposite directions as a result of the fluid action and/or they are used to bring the piston in a predetermined position with a lot of sensitivity, achieve a desired speed profile of the piston and/or more precisely adjust the resulting force applied by the piston.
In principle, a differential piston has at least two pressure surfaces, each of which is associated with a pressure chamber arranged such that the alternate action of a working fluid upon the pressure surfaces brings about a force on the piston in alternating, opposite directions. The resulting piston force is achieved as the product of the effective pressure surface and the pressure of the working fluid, while neglecting marginal variables. When simultaneously applying a force on effective pressure surfaces with opposite effective directions, the resulting piston force emerges as a differential or proper-sign addition of the partial piston forces.
If the piston, following a deflection by applying pressure on a first pressure chamber, is not returned to the starting position by an external force, that is applied by way of a mechanical spring, or if the restoring force is supposed to be adjustable, the first pressure chamber can be depressurized or the pressure thereof can be reduced and the second pressure chamber of the piston can be returned to the starting position using a defined force by way of metered pressure application.
If the pressure surfaces, arranged on opposite sides of the piston and effective for pressure application by way of a working fluid, have substantially the same size, a given pressure application on one of the two pressure surfaces of the piston results in a force, which is at least substantially equivalent to, but has an opposite effective direction to a force that would be obtained under otherwise equivalent conditions if pressure were applied to the respective other pressure surface. Such a differential piston, therefore, can be actuated almost equally in both directions through the opening and associated depressurization of one pressure chamber and the application pressure to the respective other pressure chamber.
Furthermore, differential pistons are used in order to apply relatively high resulting forces on the piston or to achieve high displacement speeds on the one hand and, if necessary, to perform even a sensitive adjustment of the resulting piston force, the piston speed and/or the piston position on the other hand. For this, in the case of large resulting piston forces at a given maximum pressure of the working fluid, a relatively large resulting piston surface is required, while in the case of small resulting piston forces, the sensitive adjustment of the piston speed or the piston position, a comparatively small resulting piston surface is advantageous. The resulting piston surface on the differential piston is obtained from the difference of the piston surfaces provided on opposite sides of the piston. Of course, it is also possible that a plurality of partial piston surfaces act additively on at least one side of the piston and/or that the effective piston surfaces change across the displacement range of the piston as a result of an appropriate geometric design.
The design-specific advantages of the differential piston, explained above, offer the possibility of bringing about high piston forces and fast piston movement as well as enabling slow piston movements, precise piston positioning and low resulting piston forces and reversing the effective direction of the piston, which means that mechanical return springs can be foregone and the restoring force can be adjusted in a simple and precise manner.
Compared to alternative solutions, further advantages are the lower number of functionally required parts, the relatively simple and space-saving design, the resulting generally lower manufacturing costs and a comparatively low total weight. As a result, differential pistons, particularly in the area of transmission technology, and in this area particularly among automatic transmissions and, in particular, among automatic transmission for motor vehicles, offer special advantages. The use of differential pistons in these areas is already known from German patent No. DE 199 32 614 A1 by the Applicant.
However, particularly in this area, the large variety of framework conditions and particularly the need for integrating a large number of components and functions in a small installation space at times produces space conditions, which previously prevented the use of differential pistons or at least made it more difficult.
In particular, a conventional design of the differential pistons in which both pressure chambers are supplied by separate fluid lines, were previously difficult or even impossible if (due to the available installation space) at least one fluid line had to be or was preferably arranged on one side of the piston, while the pressure chamber to be acted upon by this side was arranged on the other side of the piston and bypassing the piston by way of a closed loop, was not possible or desirable.
It is possible in the majority of cases to guide the end of a fluid line from a side facing away from the pressure chamber through a corresponding line bypassing the piston to a side facing the pressure chamber and connect the pressure chamber in this way to the fluid line. This, however, frequently requires considerably long lines and geometrically complicated line routing which, on one hand, causes considerable manufacturing expenses and, therefore, high costs and, on the other hand, due to the generally high specific resistance, can result in an undesirable drop in pressure and in oscillation problems, particularly in pneumatically operated systems.
Particularly in the case of differential pistons with a substantially annular shape and with pistons that are arranged on an axle or shaft, as is frequently the case in transmissions, at least one of the pressure chambers can advantageously be supplied via axial and radial fluid channels configured within the axle or shaft. However, if short line lengths are desired and existing shaft or axle diameters, according to the state of the art, are used, this is frequently only possible for a maximum of one pressure chamber of the differential piston. In addition, the access to the other pressure chamber is generally blocked by the piston itself and, therefore, can not be supplied by a simple axial bore. In this case, it was necessary until now to bypass the differential piston with more or less clearance if a supply of pressure medium from the outside was not desired.
For such a closed loop, an axial bore of an axle or shaft mounted inside the differential piston can be routed to the outside through a radial bore that is provided in a suitable location and can be connected in a sealed manner to a section of another part, for example an encompassing housing, the working fluid being directed radially outward through the bore to supply the pressure chamber with working fluid from the radially exterior side or laterally. This, however, is typically associated with the aforementioned disadvantages.
Against this background, the invention is based upon the objective of presenting a differential piston in which the supply of a pressure chamber with a working fluid can be guaranteed without complex bypass lines even if the pressure chamber to be subjected to pressure and the respective working fluid supply are arranged on opposite sides of the piston or if the working fluid supply is arranged radially inside the annular piston and a pressure chamber axially facing the piston is supposed to be supplied with pressure.